Fluid shifts between peripheral tissues and the blood stream are studied in a) man immersed in water, and b) bluefish resting on a V board in air, and tilted head up at 30 degrees or 60 degrees. A general hypothesis is being evolved, indicating that locomotion in water led to the evolution of body structure capable of withstanding hydrodynamic forces during swimming of fish. This, in turn, allowed land animals to withstand gravity without having the blood and fluid displaced downward excessively. BIBLIOGRAPHIC REFERENCES: Arthur B. DuBois. The Biological Basis for Development of Scaling Factors in Animal Locomotion. In: Scale Effects in Animal Locomotion, Edited by T. J. Pedley, Publ. Academic Press, 1977. J. Terrance Davis and Arthur B. DuBois. Immersion Diuresis in Dogs. J. Appl. Physiol, 42: 915-922, 1977.